Roof-mount air conditioners are popular with commercial properties, in part because the units can efficiently cool large spaces and in part because they don't require any modifications to existing HVAC work.
Professional A/C installers are now starting to offer these same roof units to residential customers, but do your homework before jumping on an above-ground installation. First, consider weight. While the addition of a second heat exchanger and fan doesn't substantially increase the total weight of a unit, you need to determine how much your roof can support. Commercial properties are ideal for these types of air conditioners because their roofs are often flat. If you have a sharply peaked roof or one that has damage due to weather, a rooftop unit may be a risk to avoid.
In addition, expect to pay more for a rooftop A/C unit itself and installation. Comparable roof air conditioners are anywhere from $200 to $1,000 more expensive to purchase and can cost up to double for installation. The reason? Rooftop units first need to be hauled up the side of your house, and you'll need at least several properly trained and harnessed technicians working on your roof for the better part of a day to complete the job. Installing anything two or three stories up comes with a greater safety and property risk, meaning contractors will charge more for installation.
Have you installed a rooftop A/C? Tell us about your experience in the comments below.
Editor's note: This is an updated version of an article originally posted on Aug. 26, 2013.